1. Field of the Invention
The present invention relates to a solar cell and a method of manufacturing the same, and more particularly, to a dye-sensitized solar cell (DSSC) and a method of manufacturing the same.
2. Discussion of Related Art
Nowadays, developing clean alternative energy sources is urgent due to high oil prices and environmental pollution. Among various alternative energy sources, the use of solar energy has been recognized as the most economical method, and there has been a growing interest in a solar cell configured to directly convert light energy into electric energy.
Presently, solar cells may be classified into silicon solar cells, compound solar cells, copper indium gallium selenide (CIGS) solar cells, dye-sensitized solar cells (DSSCs), and organic solar cells depending on a structure or how to operate. Among these, the first DSSC was successfully developed by Michael Gratzel, a professor at the Swiss Federal Institute of Technology in Lausanne. A DSSC may embody a transparent or semitransparent solar cell using a low-price process, generate various colors according to an organic dye, and obtain high energy conversion efficiency.
However, a thick-film liquid-electrolyte-based DSSC may have low light transmittance and be unstable due to the use of a liquid electrolyte. The low light transmittance and instability have become obstructions to commercialization of liquid-electrolyte-based DSSCs.
To solve the problems of the liquid-electrolyte-based DSSCs, Michael Gratzel, the professor at the Swiss Federal Institute of Technology in Lausanne, reported an efficiency of about 0.74% using an organic-material-based solid electrolyte instead of a liquid electrolyte in 1998 in Nature [Nature Vol 395, P 583]. Since then, a vast amount of research into DSSCs using solid electrolytes has progressed.
However, a solid-electrolyte-based solar cell has much lower energy conversion efficiency than conventional liquid-electrolyte-based solar cells and other solar cells. At present, when 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9-9′-spirobifluorene [Spiro-OMeTAD] serving as a hole transfer organic material is used as a solid electrolyte and a light absorption layer has a thickness of about 2 μm, a solar cell exhibits an energy conversion efficiency of about 5%. When the light absorption layer has a small thickness of about 2 μm, light cannot be efficiently utilized, and electrons of an inorganic semiconductor used for the light absorption layer may rapidly combine with holes of an organic semiconductor.
To overcome this drawback, a large amount of research has been conducted on increasing hole mobility of an organic material used as a hole conductor, but the efficiency is still merely about 2% to about 4%. Although there have been recent attempts at improving the thickness of a light absorption layer using TiO2 nanotubes [J. Mater. Chem., 2009, Vol 19, P 5325], the thickness of the light absorption layer is still limited to about 2 μm.